The present invention relates to saponified ethylene-vinyl acetate copolymer (hereinafter referred to briefly as EVOH) pellets and a method of producing the same. More particularly, the invention relates to EVOH pellets which are good in feedability for melt-molding (the ease with which the extrusion molding machine can be loaded with the resin), little fluctuation of machine torque and discharge amount during molding and very satisfactory in the dimensional accuracy of moldings and to a method of producing the pellets.
Generally speaking, EOVH is excellent in clarity, gas-barrier properties, aroma retention, solvent resistance, oil resistance and other properties, and by exploiting those characteristics, has been used as molded into various shaped articles inclusive of films and sheets for the packaging of foods, pharmaceuticals, industrial chemicals, agrochemicals, etc. and containers such as bottles.
Molding of EVOH pellets into such shaped articles is usually carried out by loading an extrusion or other molding machine with pellets of EVOH and melt-molding the pellets into a film or sheet but the desired EVOH moldings are sometimes not obtained depending on properties of the pellets used. For example, the deterioration of dimensional accuracy such as the accuracy of form and thickness is sometimes encountered. Therefore, the properties of pellets are important considerations. Considering the circumstances, Japanese Patent Publication (Tokkyo Koho) S47-38634 proposes a method of preparing pellets of EVOH which comprises extruding a solution of EVOH in methanol-water in the form of a strand or strands into a coagulation medium such as water or methanol-water at or below 50xc2x0 C. Kokai Tokkyo Koho S53-9898 describes a process for producing spherical EVOH pellets which comprises ejecting a slurry of EVOH from a die orifice into water or methanol-water and immediately shearing the EVOH extrudate with a cutter. Kokai Tokyo Koho S53-120767 discloses a process for producing EVOH pellets which comprises delivering a strand of coagulated EVOH through a special duct to a cutting stage. Kokai Tokkyo Koho S62-106904 describes a process for producing EVOH pellets which comprises extruding an EVOH solution supplemented with a lubricant in the form of a strand into a coagulation bath, and Kokai Tokkyo Koho H3-61507 describes a process for producing EVOH pellets which comprises transferring an EVOH strand via a belt conveyor to a cutting stage and cutting the strand under spray with water.
However, according to the technology described in Tokkyo Koho S47-38634, Kokai Tokkyo Koho S53-9898 or Kokai Tokkyo Koho S62-106904, the feedability of EVOH ill pellets for melt molding is poor, and in the technology described in Kokai Tokkyo Koho S53-120767 and Kokai Tokkyo Koho H3-61507, the comparatively uniform shape of pellets contributes to improved feedability in a certain measure but there remains some room for improvement.
The demand for improved accuracy in the form and thickness of molded articles is getting more and more stringent and the advent of EVOH pellets satisfying the above demand is awaited in earnest.
Under the circumstances, the present inventor conducted an intensive investigation and found that EVOH pellets such that the differential curve representing the magnitude of change in the cumulative pore volume curve constructed by measuring the fractional volume of surface pores by the mercury intrusion method has no peak in the pore diameter range of 1xc3x97102xcx9c1xc3x97104 xc3x85 (more preferably 1xc3x97102xcx9c5xc3x97105 xc3x85 provide a neat solution to the above problem. The present invention has come forth from the above finding.
The differential curve representing the magnitude of change in a cumulative pore volume curve as the term is used in the context of this invention means the curve representing the magnitude of change as constructed by differentiation of a cumulative pore volume curve showing the change in volume of the mercury forced into pores as measured by the mercury intrusion method.